The present invention relates to an improved technique by means of which the necessary operating clearance between the rotor of a machine and its associated stator component can be maintained at a safe minimum thereby to maintain an optimum efficiency of operation during the life of the machine.
The invention is applicable to various types of machines incorporating rotor and stator components such as for example various types of turbines, compressors and pressure wave machines, an example of the latter being disclosed in U.S. Pat. No. 3,591,313, granted July 6, 1971 to Alfred Wunsch.
The efficiency of such machines is determined by various factors including the size of the operating clearance between the rotor and stator and will decrease as the clearance increases. For this reason, efforts are being continuously made to reduce the clearance as much as possible without, however, incurring potential damage to the machinery as would be caused by accidental touching of the stator by the rotor. The optimum amount of the necessary operating clearance is determined on the basis of the properties of the materials involved, manufacturing technology, the size of the rotor and stator and also the operating conditions of the machine. Care must also be taken that in exceptional circumstances, whenever the rotor touches the stator, that damage will not occur. Such exceptional circumstances may arise, for example, during the starting-up period of the machine, or in the event of strong vibrations occurring during operation, or excessive increases in machine temperature and the like.
It has been proposed, for example, in the case of turbo-machines and compressors, to provide the tips of the rotor blading with a bevelled surface in order to keep the operating clearance between the blading and the surrounding surface of the stator to a minimum. In the event that the tips of the blades touch the stator surface, the bevelled blade will deform, or wear down, and no harmful forces will be generated.
It has also been proposed, in order to establish a minimum clearance, to coat the walls of the stator at their inner surface opposite the rotor with a relatively soft layer which can be abraded away or deformed by any accidental touching of the tips of the rotor blading. Such soft layers can consist, for example, of graphite, die-cast or sintered porous nickel-chromium alloys, or nickel-graphite materials, honeycombed cells, etc. and are called abrasable layers for touch protection. All protective layers of this type are lacking in the property to "grow" i.e. to increase in size, under the influence of the environmental temperature and/or the particular atmosphere in which the machine operates.
These known expedients have the disadvantage that the bevelled blade ends or soft layers will change their shape irreversibly upon contact, either by deforming or wearing down. In addition thereto, the blade ends, in the case of turbo-machines and compressors, or the ends of the cell walls, in the case of pressure wave machines, as well as the stator will be subjected to corrosion and erosion, with the result that the operating clearance as between rotor and stator will increase continuously and the efficiency of the machine will decrease in a corresponding manner.